DCC Gillespie has served as a policewoman for 32 years, originally with the RUC and then with the PSNI.
She climbed the ranks to become Deputy Chief Constable in 2009 and as she notes, temporary Chief Constable in the interregnum between Sir Hugh Orde and Matt Baggott.
She was considered a frontrunner to succeed Matt Baggott as Chief Constable before her retirement was announced.
"It's obviously with mixed emotions that I leave this organisation that I love, that I leave many of the fine people in it but I know that the time is right for me," she said.
"In terms of looking back over 32 years, it's been an amazing transformational process from where we were when I joined in 1982 when we patrolled routinely accompanied by the army, to where we are now today when police officers are out on the beat on foot, on bicycles and on mobile patrols in all parts of Northern Ireland. It's an amazing change."
DCC Gillespie's uncle was a serving officer, but she had no other immediate family in the RUC, so she said "it was a shock to my family and especially to my parents that I decided to join the RUC".
"It was very much out of a sense of vocation that this was about helping my community, I know that sounds very twee now, but that is exactly what it was about," she said.
"My parents were a huge influence in my life, my father always had a huge heart for his community and would've tackled criminals or youngsters who were doing damage to church premises or whatever - it kind of instilled in me a sense of community and service and that was very important."
I had exceeded my wildest dreams in what I had achieved in this organisation and it was time to move on to other things.
Judith Gillespie
Gillespie said she was turned down twice before her application to join the RUC was accepted - she was then one of four women and 86 men in training.
She said she was proud to have served in the RUC for 19 years but accepted that reform was necessary.
"Change had to be made because we couldn't continue the way we were going, there had to be some changes and yes some of them were very controversial, 50-50 recruitment for example, was very controversial and the name change."
DCC Gillespie said in her three decades on duty she has noted a significant change in support for Northern Ireland's police service, especially from the political world.
"They mightn't always agree with every decision that the PSNI makes but at the heart is the support for good policing and I think that this has changed immensely from when I joined the RUC," she added.
"I think there is still a way to go in certain communities, both loyalist and republican communities, we've still a way to go in building confidence but we're on the right road and right vision."
DCC Gillespie reflected on negative attention and headlines the PSNI has received, particularly in the wake of a shocking video showing a woman being left at the side of the road by police officers in Londonderry.
Policing is a very human endeavour and of course in this world of immediate social media and live photography, that's very unforgiving but I think good policing is at the heart of all that we do.
Judith Gillespie
"Despite the best selection processes to get the best people into the organisation, despite world-class training, despite very good supervision and very good equipment, police officers will from time to time make mistakes, they will make errors of judgement, they will make errors of fact and they may do ill-judged things," she said.
"Because policing is an organisation comprised of human beings and human beings are not perfect, myself included. There were times when I've made mistakes too.
"Accountability is about holding police officers that get it wrong to account on some occasions where it's appropriate, saying sorry when you get it wrong as an organisation. That has led to an increase in public confidence, the increased accountability and oversight that we have through an independent ombudsman and Policing Board."
It was at the end of the World Police and Fire Games last year that she made her mind up to retire, a decision she stuck to despite knowing about Justice Minister David Ford's proposals to change the criteria for the top job.
She said: "How could I top this experience?"
But the deputy chief said she was "absolutely confident" there will eventually be a woman in charge of the PSNI.
She added: "It might be sooner than you expect because as the Policing Board advertises for a new chief we may attract female applicants from England, Wales or Scotland, so you just don't know.
"One thing I'm sure of though, the Policing Board will make the right decision, they made the right decision five years ago by appointing me as Deputy Chief Constable. I think they'll select the right person, the best person whether they're male or female."